As is known, the technique of printing using lithographic plates is based on a difference in distribution between fatty substances and water. The fatty substance, or ink, is retained by the image area and water is retained by the non-image area. When the surface of a suitably prepared lithographic plate is moistened with water and then sprayed with ink, the non-image area retains the water and rejects the ink, while the image area accepts the ink and rejects the water. The ink is then transferred from the image area to the surface of a material on which it is desired to reproduce the image, such as for example paper, fabric and the like.
In general the lithographic plates used in printing processes are made from an aluminium substrate coated with a composition which is sensitive to light radiation and/or heat.
In plates of the conventional type the stage of exposure to light radiation and/or heat is followed by a stage of developing in an aqueous alkaline bath. When the portion of the exposed composition is soluble in the developer bath, the printing process is known as “positive”. Conversely, when the exposed portion is insoluble, the printing process is known as “negative”. In both cases the remaining image area is lipophilic and therefore accepts the ink, while the non-image area is hydrophilic and accepts the water.
EP-A-0 924 065 describes a heat-sensitive member forming an image without the ablation of material for a lithographic plate having as an upper layer on a substrate an image-forming layer comprising a binder which is switchable to heat, characterised in that the said image-forming layer becomes hydrophobic under the influence of heat, the said binder which is switchable to heat having attached hydrophilic groups and being a (co)polymer containing monomer units selected from the group comprising malic acid, itaconic acid, 3- or 4-vinylphthalic acid, cis-1,2,3,6-tetrahydrophthalic acid cis-norbene-endo-2,3-dicarboxylic acid and their hemiesters.
The heat required to form the image is provided by IR radiation and preferably the image-forming coating also contains a dye (absorber) which is capable of absorbing IR radiation and converting it at least partly into heat.
The plate in EP-A-0 924 065 has the advantage that it does not require any developing treatment after the stage of exposure to heat. In addition to this the equipment used to expose this plate to heat does not require those special systems for the collection of removed wastes which are required in the case of ablation plates, where the image is formed by removing the parts of the coating which have been exposed to heat.
This does however have the disadvantage that it requires the application of high energy in order to form the image. In fact Examples 1 and 2 in EP-A-0 924 065 indicate that in order to form an image using a laser with a power of 11 watts the drum must rotate at a speed of 40 rpm. This is equivalent to energy of approximately 710 mJoule/cm2. In addition to the expenditure of energy, this means that the machine must be used at low speed, with consequent low productivity.
This makes such a plate unacceptable for the graphic arts industry.
In addition to this, experiments which have been performed by the Applicant have demonstrated that a lithographic plate coated with the composition described in Example 1 of the aforesaid patent has some affinity for ink even in the regions which have not been exposed. It follows that the images printed do not have white backgrounds (Comparison Example 1).
Another disadvantage of the aforesaid composition is that the heat-switchable binder which it contains is not stable over time. In fact in addition to incorporating a number of attached carboxyl groups, the said binder also comprises a certain percentage of maleic anhydride rings, and the latter tend to open over time, thus changing the properties of the heat-switchable binder.
The said heat-switchable binder is stable over time only when all the malic anhydride rings have been opened to form attached carboxyl groups. The Applicant has therefore tested a composition similar to that described in Example 1 in the aforesaid patent, but using a heat-switchable binder in which the malic anhydride rings have been opened by acid hydrolysis. This plate did not however form an image which had an affinity for ink.
In the graphic arts industry there is therefore still a greatly felt need for a negative lithographic plate which does not have the disadvantages of the plate in EP-A-0 924 065.